Data processing system employing quench simulation for enabling accurate computation of sample activity levels in liquid scintillation spectrometry

ABSTRACT

A data processing system for scintillation spectrometers of the type for measuring activity levels of samples containing radioactive isotopes and subjected to varying degrees of quench, including methods and apparatus for imposing a controlled simulated quench condition on each sample which, when added to the actual internal quench condition of the sample, creates an effective quench condition equal to a known predetermined actual quench condition for which counting efficiency is known with a high degree of accuracy, whereby true activity levels can be accurately computed in decay events per minute without incurring statistical errors inherent in interpolation and extrapolation techniques employed with conventional quench correlation data. Various methods and apparatus are described for creating a controlled simulated quench for each sample, together with an automatic computational system which permits direct display of activity levels in units of decay events per minute.

United States yatent Cavanaugh, .Fr.

[ 51 Sept. 12,1972

[ DATA PROCESSING SYSTEM EMPLOYING QUENCH SIMULATION FOR ENABLINGACCURATE COMPUTATION OF SAMPLE ACTIVITY LEVELS IN LIQUID SCINTILLATIONSPECTROMETRY [72] Inventor: Robert E. Cavanaugh, In, La

Grange Park, Ill. [73] Assignee: Packard Instrument Company, Inc.,

Downers Grove, Ill.

[22] Filed: April 14, 1967 [21] Appl. No.: 630,891

[52] US. Cl. ...250/106 SC, 235/15l.35, 250/71.5 R, 250/83 C, 250/207,313/153 [51] Int. Cl ..G01t 1/20 [58] Field of Search...235/151 35;250/106 SC, 71.5, 250/207, 83 C; 313/153 [56] References Cited UNITEDSTATES PATENTS 2,213,769 9/ 1940 Ruska ..250/207 2,772,368 11/1956Scherbatskoy ..250/207 3,101,409 8/1963 Fite ..250/7l.5 3,188,468 6/1965Packard et a1 ..250/71.5 3,381,130 4/1968 Nather ..250/7l.5

Comunnetti ..250/7 1.5 Farnsworth ..250/207 [57] ABSTRACT A dataprocessing system for scintillation spectrometers of the type formeasuring activity levels of samples containing radioactive isotopes andsubjected to varying degrees of quench, including methods and apparatusfor imposing a controlled simulated quench condition on each samplewhich, when added to the actual internal quench condition of the sample,creates an effective quench condition equal to a known predeterminedactual quench condition for which counting efliciency is known with ahigh degree of accuracy, whereby true activity levels can be accuratelycomputed in decay events per minute without incurring statistical errorsinherent in interpolation and extrapolation techniques employed withconventional quench correlation data. Various methods and apparatus aredescribed for creating a controlled simulated quench for each sample,together with an automatic computational system which permits directdisplay of activity levels in units of decay events per minute.

29 Claims, 24 Drawing Figures PATENTED SEP 12 m2 SHEEI BEEF 16 [re/0emay im zr A? (mu 4144 f6 PATENTED EHBW 3.691.386 SHEET OBUF 16 SHEETUSUF 16 PATENTED SE? 12 I972 PATENTED E! 1 2 I SHEET UBUF 16 N M M g j z1 WUWL w; wllm W WE M .hlhlw Q G Q Q Q Q Q Q Q E Q G mu llll fix m Z aV4 MM r W. M w m/ 1 WW DATA PROCESSING SYSTEM EMPLOYING QUENCHSIMULATION FOR ENABLING ACCURATE COMPUTATION OF SAMPLE ACTIVITY LEVELSIN LIQUID SCINTILLATION SPECTROMETRY CROSS REFERENCE TO RELATEDAPPLICATIONS Robert E. Cavanaugh, Jr., Ser. No. 54l,72l, filed Apr, 11,1966 now US. Pat. No. 3,499,149.

Stanley M. Bristol, Ser. No. 629,462, filed Apr. 10, 1967.

Lyle E. Packard, Ser. No. 630,892, filed Apr. 14, 1967.

BACKGROUND OF THE INVENTION I The present invention relates in generalto liquid scintillation spectral analysis of test samples containingunknown isotopes disposed in a liquid scintillator and, moreparticularly, to spectral analysis equipment and techniques which may bewholly automatic in operation, which virtually eliminate the effects ofquenching as a source of error in determining true activity levels oftest samples, and which readily permit direct display of such trueactivity levels, either visually or in printed form, in units of decayevents per minute (dpm). In its principal aspects, the invention isconcerned with an improved data processing system, as well as withimproved methods and apparatus for simulating a controlled quenchcondition for all test samples, irrespective of whether the latter areactually unquenched or quenched and irrespective ofthe degree to whichsuch samples may actually be quenched, whereby the effective quenchcondition of the sample (viz., the amount of actual quench internally ofany given sample plus the simulated quench condition superimposedthereon externally of the sample) converges to a known selected valuefor which instrument counting efficiency is known with a high degree ofaccuracy.

Modern apparatus for detecting and measuring radioactivity has reachedan unusually high state of development with systems currently availablewhich offer unusual sensitivity to low energy radiation, as well asvarious options of full automation, semi-automation, or the moreeconomical manual operating version, In a relatively few years, greatstrides have been made towards improving the preciseness and accuracy ofcounting efficiency in compliance with the very stringent requirementsof users of this highly technical and sophisticated equipment. However,certain problems have continued to plague both the manufacturers andusers of such equipment. A particularly prevalent and vexing problem hasbeen the error introduced into computations of true sample activitylevels because of a phenomenon commonly encountered with liquidscintillation samples known as quenching. Stated very simply, thisphenomenon results in attenuation of light scintillations within thesamples, thus significantly affecting the statistical accuracy of theequipment which determines activity levels based upon the number andenergy of such light scintillations, the latter being counted over knownunits of time and being proportional in energy to the energy of thedecay events which produce them.

Many efforts have heretofore been made to minimize and, preferably, toeliminate, the errors which result from the quench phenomenon, some ofwhich have completely failed and others of which have met with varyingdegrees of success and acceptance-however,

notwithstanding all such efforts, the problem has remained as a sourceof error, which ofttimes amounts to significant error in the computationof true activity levels.

One principal effort that has heretofore been made towards minimizingthe quench problem has been that of development various constituentswhich make up the sample and which are as free of quench characteristicsas possible. Such constituents include, without limitation thereto,scintillation substances, solvents, and the material from which thelight transmissive sample vial is made. However, perfect lighttransmitters completely devoid of quench characteristics are simply notavailable, and even if they were, the problem would remain since thetest specimen itself may, and often will, contain quench materials such,for example, as blood or urine, which tend to attenuate the lightbecause of their color. Moreover, unless the detection system ismaintained in a completely enclosed atmosphere of an inert gas such asargon, quench can occur simply because of the presence of air.

Faced with the seeming impossibility of eliminating the quenchphenomenon as a source of error, numerous efforts have been made to copewith the problem by providing methods and apparatus for compensating forsuch errors. Typical systems which are currently employed and which havefound great acceptance today by people employing this sophisticatedequipment include systems in which an external standard source whichemits highly penetrating radiations is placed adjacent the sample in thedetection chamber during a portion only of its overall counting cycle.Light scintillations occurring in the sample are then counted during atleast two discrete intervals, during one of which the scintillations arecreated only by the isotope in the sample and during the other of whichthe scintillations are created by the composite effect of the isotopeand the external standard. Suitable electronic equipment is provided forseparating the pulses from the two sources on the basis of theirdifferent energy levels and, therefore, those which are counted primarily from the external standard provide a fairly accurate indication ofthe degree of quenching present in the sample since the countingefficiency for such external standard is known or can be readilyascertained by use of an unquenched standard sample. Typical systems ofthis type are described in detail in Lyle E. Packard US. Pat. No.3,188,468, issued June 8, 1965, as well as in the aforesaid Cavanaughapplication Ser. No. 541,721, filed Apr. 11, 1966, both of which areassigned to the assignee of the present invention.

Both of the aforementioned prior systems are of the type which arecommonly referred to as external standardization systems and bothrepresent basic and significant improvements over earlier known systemsdescribed therein, such as internal standardization" and channels ratio"systems. In effect, however, all of these prior systems have had certainaspects which are common to one another, a principal one of which isthat the measured quench correlation parameter (e.g.,

net external standard count, external standard ratio," channels ratio,"etc.) generally provides an indication of the degree of quench presentin the sample, which indication must then be compared with a previouslyprepared quench correlation curve in order to determine the countingefficiency. Once knowing the counting efficiency, the counts per minute(cpm) measured for the isotope being analyzed can be divided by countingefficiency to determine activity level in decay events per minute (dpm).Unfortunately, however, the quench correlation curve itself differswidely from instrument to instrument, from isotope to isotope, fromchannel to channel, with sample volume, and with other variableconditions. Consequently, it has heretofore been necessary to preparemany of such curves, the preparation of each one of which has beentimeconsuming, expensive, and subject to numerous human errors. Moreover,once the curves are prepared, it is necessary that the measured quenchcorrelation data be compared with them in order to determine countingefficiency, thus introducing even further danger of human error.

Even more significant, however, has been the fact that while such acorrelation curve can be prepared, it is only as accurate as the numberof points which actually define the curve. It has been established thatsuch points simply do not fall on a straight line, or even on a smoothlycurved line-quite to the contrary, the points will be non-uniformlydistributed in an unpredictable random pattern which only generallydefines the correlation curve. Consequently, even when the technicianuses extreme care in his computations, he has been forced to extrapolateor interpolate between known points and, since the extrapolated orinterpolated data can vary significantly from the actual data, thecomputed efficiency can still vary greatly from actual efficiency withmaximum errors on the order of up to percent and average errors on theorder ofup to 2 percent being common, dependent upon the number ofdifferently quenched standard samples selected to prepare the quenchcorrelation curve.

Errors of the foregoing magnitude are simplynot acceptable to the highlytrained technical personnel who use this general type of equipment.Indeed, such errors are highly objectionable, and the more so in view ofthe high state of sophistication that the overall art has reached.However, these errors have heretofore been tolerated only because theprior systems briefly described above, and described in considerablygreater detail in the aforesaid Packard US. Pat. No. 3,188,468 andCavanaugh application Ser. No. 541,721, have represented the bestavailable solutions to the problem up until the present date.

Continued efforts have, however, been made towards providing a moresatisfactory solution to the problem. For example, it has been suggestedthat true activity level for a sample can be computed simply by dividingthe measured variable quench correlation parameter (e.g., externalstandard ratio, net external standard count, channels ratio, etc.) intothe measured value in counts per minute (cpm) for the sample undergoinganalysis. This suggestion, however, is not satisfactory for manyreasons. First, it assumes that the quench correlation curve is astraight line, which it is not. Secondly, it fails to take into accountthe non-uniform random distribution of points which define such a curve.Therefore, even were the curve a straight line or substantially astraight line, errors of the same general magnitude as described abovewould still be experienced. The fact that the quench correlation curveis not a straight line actually adds to the magnitude of such errorswith the result that errors on the order of up to 25 percent can be, andhave been, experienced.

It has also been proposed that the problem can be resolved by adjustingin any of various known manners, overall system gain so as to restorethe measurable quench correlation parameter to a value indicative of anunquenched sample, and thereafter, analyzing the sample as if it wereunquenched. Again, however, such a proposed solution is no solution atall since the gain correlation curves do not coincide with nor followthe quench correlation curves and, consequently, the magnitude of errorcan be and often will be, even greater than that experienced with theinterpolation/extrapolation techniques referred to above.

It is a general aim of the present invention to provide an improved dataprocessing system which overcomes the foregoing disadvantages and whichis characterized by its reliability and rapidity of operation. In thisconnection, it is an object of the invention to provide improvedradioactivity spectrometry methods and apparatus which permit thedetermination of activity levels for test samples having any of a widerange of different quench characteristics with virtually unlimitedaccuracy, yet wherein this is accomplished by the utilization of quenchcorrelation data based upon the measurement of only a relatively few,and indeed, in some instances, only one, known standards or standard.While not so limited in its application, the invention will findespecially advantageous use when the measured variable parameter of suchquench correlation data takes the form of net external standardizationratios, although it can also be used in connection with other measurablevariable parameters which also provide an indication of the degree ofquenching such, merely by way of example, as channels ratios, or netexternal standard counts.

As a consequence of attaining the foregoing general objective of theinvention, it has been found that ancillary benefits achieved are thatthe danger of human error in both the preparation and reading of quenchcorrelation data is greatly reduced; the versatility of such data issignificantly increased; and the time and cost required to prepare suchdata are held to a minimum.

In another of its important aspects, it is an object of the invention toprovide improved methods and apparatus suitable for use in radioactivityspectrometry applications which permit highly accurate determination ofsample activity levels in terms of decay events per minute (dpm), andwhere such information can be printed or read directly without requiringthe technician to perform close and tedious comparisons of detected ormeasured data with quench correlation curves.

It is a related object of the invention to provide improved spectrometrymethods and apparatus which will greatly facilitate and speed up thequantitative determination of activity levels in terms of decay eventper minute (dpm), yet wherein this is not only accomplished withoutsacrificing accuracy of the computations but, to the contrary, whereinthe measurements are, on an average, considerably more accurate than hasheretofore been feasible on a commercial basis.

It is one of the principal objectives of the present invention toprovide novel spectrometry methods and apparatus for determiningactivity levels of test samples wherein provision is made forautomatically compensating for errors attributable to quenching so as topermit direct display of activity levels in units of decay events perminute (dpm), either visual display or printed display, all irrespectiveof the amount of quenching that may be present in any given sample.

Another general aim of the present invention is the provision of noveland improved methods and apparatus suitable for use in computing trueactivity levels, corrected for quench errors, of test samples in unitsof decay events per minute (dpm) which are characterized by theiraccuracy and by their complete elimination of any need to visually orautomatically interpolate or extrapolate between known points on aquench correlation curve, which interpolation or extrapolation hasheretofore been essential and which has inherently introducedsignificant errors into activity level computations.

A further object of the invention is the provision of novel methods andapparatus for producing simulated quench conditions for each test sampleundergoing examination so as to bring the effective quench level of thesample (the effective quench level consisting of the actual internalquench level of the sample plus the superimposed external simulatedquench level) to a known and selectable level for which countingefficiency is accurately known, yet wherein this is accomplished withoutany noticeable loss in statistical counting accuracy.

It is a more specific object of the invention to provide novel methodsand apparatus for statistically modulating signals representative ofdecay events in each test sample so as to simulate the presence of aquenching agent in the sample, and for adjusting the modulating signalsso as to cause the effective quench level of the sample to converge upona particular one of a plurality of preselected quench levels for each ofwhich the counting efficiency is known with a high degree of accuracy.In this connection, it is an object of the invention to simulatequenching for all test samples, whether the latter are quenched orunquenched, and irrespective of the degree of quenching that may bepresent, so as to bring the particular variable quench correlationparameter being measured (i.e., external standard ratio, net externalstandard count; channels ratio; etc.) from its actual value for anygiven sample to a selected one of a plurality of preselected simulatedvalues for each of which counting efficiency is known with a high degreeof accuracy.

It is a more detailed object of the invention to provide novel methodsand apparatus for statistically modulating the photoelectron energy of aconventional photon-to-electron energy transducer such as aphotomultiplier, primarily in the region of the cathode and/or firstdynode where counting statistics can be affected, preferably bymodulating the quantum efficiency of the photomultiplier or bymodulating the collection efficiency of the first dynode, so as tosimulate a quench condition for the sample equivalent to that producedby the presence of an actual quenching agent in the sample itself, whichsimulated quench condition is automatically controlled so as to adjustthe effective quench level for any given sample (irrespective of itsactual quench level) to a selectable one of a plurality of preselectedpoints at each of which the counting efficiency is known with a highdegree of accuracy.

An ancillary object of the invention is the provision of novel methodsand apparatus for establishing quench correlation data rapidly,economically, and with a high degree of accuracy, yet without requiringany advance preparation of standards having known and different quenchconditions, and without requiring repetitive manipulation of suchstandards, either manually or automatically, and wherein an unlimitednumber of simulated quench conditions can be produced using only asingle unquenched standard.

Other objects and advantages of the invention will become apparent asthe following description proceeds, taken in conjunction with theaccompanying drawings, in which:

FIG. 1 is a fragmentary side elevational view, partly in section,depicting an exemplary radiation detection chamber and elevatormechanism suitable for processing samples in accordance with the presentinvention, the apparatus here being depicted with the elevator mechanismin the down or sample loaded position with the sample to be analyzedinterposed between a pair of light transducers;

FIG. 2 is an enlarged fragmentary side elevational view, partly insection and partly in diagrammatic form, here showing a conventionalapparatus for selectively positioning and recirculating externalstandard radioactive source material between a first position remotefrom and shielded from the detection chamber and a second positionadjacent a sample disposed in the detection chamber;

FIG. 3 is a fragmentary schematic wiring diagram of the controlcomponents utilized for positioning standard radioactive source materialin a selectable one of two positions in accordance with the particularcycle of operation determined by the mode program control, the latterbeing depicted partially in block and partially in diagrammatic form;

FIG. 4 is a schematic wiring diagram, partly in block form, heredepicting an exemplary control system for operating the elevatormechanism shown in FIGS. 1 and 2, together with a conventionalelectrical system which accepts, counts and records the output of theradiation detector while at the same time providing certain controlinputs in accordance with the present invention for the exemplarycomputer program control shown in FIG. 15;

FIG. 5 is an elevational view of a typical master control panel suitablefor use with apparatus embodying the features of the present inventionand capable of carrying out the methods of the present invention;

FIG. 6 is an elevational view, partly in diagrammatic form, of anexemplary auxiliary control panel by which the technician mayselectively dial in predetermined efficiency values, or numericalrepresentations thereof, for one or more isotopes in different pulseheight analyzing channels so as to enable direct display of

1. Apparatus for simulating quenching in test samples containing one ormore radioactive isotopes disposed in a liquid scintillator comprising alight transducer, means for positioning a sample adjacent saidtransducer in light transmissive relationship therewith so as toestablish a sample-transducer combination which permits counting oflight scintillations occurring in the sample, and means for controllablyaltering the quantum efficiency of the transducer so as to affectcounting statistics of said sample-transducer combination and thussimulate quenching in said sample.
 2. ApparatUs as set forth in claim 1further characterized in that said transducer comprises aphotomultiplier.
 3. Apparatus as set forth in claim 2 furthercharacterized in that means are provided for reducing the collectionefficiency of the first dynode of said photomultiplier.
 4. Apparatus asset forth in claim 2 further characterized in that said means foraltering the quantum efficiency of said photomultiplier comprises acontrollable magnetic field.
 5. Apparatus for simulating quenchconditions for a sample containing at least one radioactive isotopedisposed in a liquid scintillation medium comprising, in combination, aphotomultiplier for detecting light scintillations in the sample, meansincluding said photomultiplier for measuring a selected quenchcorrelation parameter for the sample, means for establishing a variablesimulated quench modulating signal having a value which is a function ofthe measured quench correlation parameter, and means for controlling thevariable simulated quench modulating signal so as to vary the quantumefficiency of said photomultiplier and thus affect counting statisticsto cause a measured selected quench correlation parameter for the sampleto shift towards a fixed preselected point.
 6. Apparatus for simulatingquench for samples containing one or more radioactive isotopes disposedin a liquid scintillation medium comprising a photomultiplier, means forestablishing an electromagnetic field at the cathode/first dynode regionof the photomultiplier, a source of current for said means, and meansfor varying the current flow in said field establishing means so as tosimulate quenching by controllably altering counting statistics at saidregion in accordance with the equation where I equals the currentflowing in said means and R equals a measurable quench correlationparameter for said sample indicative of the effective quench level forthe sample.
 7. Apparatus as set forth in claim 6 further characterizedin that said field establishing means comprises a coil surrounding saidregion.
 8. Apparatus as set forth in claim 7 further characterized inthat said parameter is channels ratio.
 9. Apparatus as set forth inclaim 7 further characterized in that said parameter is net externalstandard ratio.
 10. Apparatus as set forth in claim 7 furthercharacterized in that said parameter is net external standard count. 11.A data processing system for determining activity levels of test samplescontaining one or more radioactive isotopes in units of decay events perunit time and wherein counting efficiency for the system is known onlyat n discrete fixed points for any given isotope and wherein ameasurable quench correlation parameter R for the sample is known ateach of said points, said system comprising, in combination, first meansfor storing a numerical representation of counting efficiency for aknown value of the parameter R corresponding to at least one of the ndiscrete points, second means including a photomultiplier for measuringthe parameter R for each sample, third means for comparing the measuredparameter R for the sample with the known value of parameter R, fourthmeans for varying the quantum efficiency of said photomultiplier so asto simulate quenching for each sample, fifth mans for controllablyvarying said fourth means to cause the measured parameter R to convergeupon the known value of the parameter R, sixth means for counting lightscintillations per unit time created in the sample by isotope decayevents therein and for producing a numerical representation of countsper unit time, and seventh means for dividing the stored numericalrepresentation of counting efficiency into the numerical representationof counts per unit time which are produced after said measured parameterR has converged upon the known value of the parameter R.
 12. A dataprocessing system as set forth in claim 11 further characterized in thatsaid fourth means comprises meaNs for establishing a magnetic field atthe cathode/first dynode region of said photomultiplier.
 13. A dataprocessing system as set forth in claim 12 further characterized in thatsaid fifth means comprises means for shifting said magnetic fieldrelative to the cathode/first dynode region of said photomultiplier. 14.A data processing system as set forth in claim 12 further characterizedin that said fifth means comprises means for changing the strength ofsaid magnetic field.
 15. A data processing system as set forth in claim11 further characterized in that said fourth means comprises a coil anda current source for said coil.
 16. A data processing system as setforth in claim 15 further characterized in that said coil is concentricwith said photomultiplier and is disposed at the cathode/first dynoderegion thereof.
 17. A data processing system as set forth in claim 16further characterized in that said fifth means is responsive to themeasured parameter R.
 18. A data processing system as set forth in claim11 further characterized in that said fourth means comprises means forvarying the electrostatic field at said cathode/first dynode region. 19.A data processing system as set forth in claim 18 further characterizedin that said fifth means is responsive to said measured parameter R. 20.A data processing system as set forth in claim 11 further characterizedin that said fourth means comprises means for varying the potentialdifference between said cathode and said first dynode where countingstatistics can be affected while leaving the potential levels at allother dynodes unchanged.
 21. A data processing system as set forth inclaim 20 further characterized in that means are provided forincrementally raising the cathode potential towards the first dynodepotential in response to the value of the measured parameter R.
 22. Adata processing system as set forth in claim 20 further characterized inthat means are provided for incrementally decreasing the first dynodepotential relative to the cathode potential while leaving the potentialat all other dynodes unchanged in response to the value of the measuredparameter R.
 23. A data processing system as set forth in claim 11further characterized in that said first means comprises means forstoring numerical representations of counting efficiencies for the knownvalues of the parameter R corresponding to a plurality of discretepoints, and wherein said fifth means causes the measured parameter R toconverge upon the next lower known value of the parameter R, and saidseventh means includes means for selecting the stored numericalrepresentation of counting efficiency corresponding to the known valueof the parameter R to which the measured parameter R has converged. 24.For use with a radiation detection system of the type employingsimulated quenching to produce an effective quench level for a testsample containing one or more radioactive isotopes and which issubjected to quenching; a method for compensating for errors produced bysuch quenching comprising the steps of: a. establishing a fixed quenchcorrelation parameter for which counting efficiency is known, b.monitoring light scintillations produced in the sample with aphotomultiplier, c. establishing a controllable magnetic field at thecathode/first dynode region of the photomultiplier, d. measuring theeffective quench correlation parameter, and e. causing variations in thefield effect upon electrons in the cathode/first dynode region of thephotomultiplier so as to affect counting statistics and thus simulatequenching to shift the measured quench correlation parameter for thesample to a level corresponding to the established fixed quenchcorrelation parameter.
 25. A method as set forth in claim 24 furthercharacterized in that said field is established by a coil coupled to acurrent source and the current flowing in said coil is varied as afunction of the value of the measured quench cOrrelation parameter. 26.A method as set forth in claim 25 further characterized in that saidparameter is net external standard ratio.
 27. For use with a radiationdetection system of the type employing simulated quenching to produce aneffective quench level for a plurality of test samples each containingone or more radioactive isotopes and which are subjected to varyingdegrees of quenching; a method for compensating for errors produced bysuch quenching comprising the steps of a. establishing a plurality offixed quench correlation parameters for each of which countingefficiency is known, b. monitoring light scintillations produced in eachsample with a photomultiplier, c. establishing a controllable magneticfield at the cathode/first dynode region of the photomultiplier, d.measuring the actual quench correlation parameter for each sample, e.causing variations in the field effect upon electrons in thecathode/first dynode region of the photomultiplier so as to affectcounting statistics and thus simulate quenching to shift the measuredquench correlation parameter for each sample to a level corresponding tothe next lower one of the established fixed quench correlationparameters.
 28. A method as set forth in claim 27 further characterizedin that said field is an axial field.
 29. A method as set forth in claim27 further characterized in that said field is varied as a function ofthe measured quench correlation parameter.